Head mounted display systems have been developed for a number of different applications including use by aircraft pilots and for simulation. Head mounted displays are generally limited by their resolution and by their size and weight. Existing displays have relatively low resolution and are positioned at a relatively large distance from the eye. Of particular importance, is to keep the center of gravity of the display from extending upward and forward from the center of gravity of the head and neck of the wearer, where it will place a large torque on the wearer""s neck and may bump into other instruments during use. There is a continuing need to present images to the wearer of a helmet mounted display in a high-resolution format similar to that of a computer monitor. The display needs to be as non-intrusive as possible, leading to the need for a lightweight and compact system.
Head mounted displays can also utilize eye tracking systems in flight control, flight simulation and virtual imaging displays. Eye control systems generate information based on the position of the eye with respect to an image on a display. This information is useful for a variety of applications. It can be used to enable the viewer to control xe2x80x9chands-freexe2x80x9d movement of a cursor, such as a cross-hair on the display.
Apparatus for detecting the orientation of the eye or determining its line-of-sight (LOS) are called occulometers or eye trackers and are well known in the art. (See for example U.S. Pat. Nos. 4,109,145, 4,034,401 and 4,028,725).
In accordance with the present invention a head mounted display is preferably either an electroluminescent (EL) or an active matrix liquid crystal display (AMLCD) comprising thin film transistor (TFT) driving elements formed of single crystal silicon and then transferred to a transparent glass substrate. Each TFT circuit is connected to an electrode which defines a picture element (pixel) of the display. The head mounted display system can also include a detector array comprising thin film integrated optical diode detectors is formed of III-V materials and transferred directly onto a flat panel active matrix display.
In a preferred embodiment of a direct view eye tracking dispaly, the detectors are positioned such that each is completely above the drive transistors of the active matrix circuit i.e., adjacent to the pixel area and therefore do not block any of the display""s light output. The light output from the display, either infrared or visible, is used to determine the position of the eye. No additional optics, such as, fiber optics to/from remote displays are required in this approach. The chief advantage is that the integrated eyetracker/display can be inserted in a helmet-mounted optical system without physical modification to the helmet or optics. This advantage results from the fundamental reciprocity of the axial light rays that are used to determine the eye position. An axial ray, is a light ray that emanates from the display and travels through the optical axis of the eye, normal to the retina. These rays, when reflected by the retina, can travel back to the display along the same optical path (in accordance with the optical reciprocity theorem). Except for divergence of the rays, the reflected rays return to the vicinity of the emitting pixel. In this way, the detector can identify the area of the display that is sighted by the user. Software in a computer then provides a cursor at this location.
In another alternative embodiment, instead of using the visible scene from the display, some of the frames in the display are used for brief presentation of an interlaced eyetracker pattern. If the repetition rate of the test pattern is sufficiently infrequent, the user (viewer) will not perceive its presence. This pattern can consist of a single pixel being illuminated or can have some other geometric pattern. Light from a single lit pixel enters the eye through the pupil and is reflected from the retina. The path of the reflected light clearly depends on the position of the eye. On the reverse path back to the display panel, the reflected light undergoes spreading or convergence depending upon the optical system. As it returns to the plane of the display, it strikes the photodetectors. A pattern will appear in the output of the photodetector array that depends on the position of the eye and the nature of the optical system. This pattern is interpreted by a computer and correlated to the position of the eye.
The present invention uses a single-crystal material to produce a high-density active matrix array in a head mounted optical support system that provides for closeness of the display to the eye, compactness of the array and provides the desired level of resolution. With a density of 400 lines per centimeter, for example, a 1.27 centimeters display in accordance with the invention will fit into a system only 1.52 centimeters in depth. This system is more compact, has lighter weight, and a lower cost than existing head mounted displays.
To get the display system as close as possible to the eye and as compact as possible, a short focal length lens system must be used. The focal lengths of simple lenses are limited by lens geometry, where the thickness of the lens is less than the lens diameter. Thus, a simple lens has a shorter focal length as well as a small diameter. For the most compact system, the smallest possible lens that focuses the display image is used. The lens size is defined by the object size, which in this case is the size of the display element.
Since resolution needs to be increased while size needs to be decreased, the pixel density of the display needs to increase. Existing displays have pixel densities of about 120 lines per centimeter and are about 4.1 centimeters in diameter. Using a 3.81 centimeter lens, where the minimum focal length for a standard 3.81 centimeter lens is about 3.05 centimeters, results in a lens with a center thickness of over 1.52 centimeters. The use of this lens results in a lens-to-display distance of about 3.3 centimeters, which is the minimum depth of an existing head-mounted display for this geometry.
The present system, by increasing the pixel density to at least 200 lines per centimeter, and preferably to over 400 lines per centimeter, provides for a lens-to-display distance of less than one inch.
The lens-to-display distance is preferably in the range of 1.0-2.2 centimeters.
The display can be a transmission type display with the light source directly adjacent the light valve active matrix or the light source can be positioned above the head or to one or both sides of the head of the user such that the light can be coupled to the light valve active matrix by one or more reflective elements. Fiber optics can also be employed to provide a back light source for the display or to deliver images from the display into the user""s field of view.
Alternatively, the display can be an emission type device such as an active matrix electroluminescent display or an active matrix of light emitting diodes (LEDs).
Additional embodiments of the invention include a projected view active matrix display in which different polarization components of light are separated, one component being directed to the left eye, and another component being directed to the right eye. This provides a more efficient optical system in which more light from the source is used to provide the desired image.
Another preferred embodiment utilizes an active matrix display in which the pixel size increases across the display to provide a wide angle field of view display.
The display can be fabricated as a visor with a number of displays which are tiled together and positioned on a flat or curved plastic visor.